A non-volatile semiconductor memory device preserves stored data even when power is interrupted. Various types of memory cells suitable for non-volatile semiconductor memory devices are known, one of which is a single transistor type memory cell.
Generally, a single transistor type memory cell MC, as illustrated in FIG. 1, includes: a current channel between source and drain regions of a semiconductor substrate; a floating gate FG between a dielectric oxide DOX and a gate oxide GOX; and a control gate CG. The floating gate FG traps electrons, and the trapped electrons establish a threshold voltage. When the non-volatile semiconductor memory device performs a read operation, the threshold voltage of the memory cell is sensed, so that stored data may be recognized.
Conventionally, program and erase operations may be repeatedly performed in the memory cells of a non-volatile semiconductor memory device. At this time, various functions of a single transistor memory cell MC may be determined by various types of supplied voltages. In the single transistor memory cell MC, electrons move into a floating gate FG to program the single transistor memory cell. The electrons that move into the floating gate FG may be generated by Fowler-Nordheim tunneling FN, electron injection, or the like. Electron injection may be performed by Channel Hot-Electron injection or Channel-Initiated Secondary Electron Injection. Furthermore, Fowler-Nordheim tunneling is widely used in flash memory devices to erase all data at once.
Conventionally, a transistor memory cell stores one of two data values. The two data values may be determined by a threshold voltage which is set to one of two levels, as illustrated in FIG. 2. For example, when the threshold voltage of a memory cell is less than a reference voltage VM, data is read as “1”, and when the threshold voltage of the memory cell is greater than the reference voltage VM, data is read as “0”.
To provide increased integration of semiconductor memory devices, 4-level memory cells have been developed. A 4-level memory cell may be programmed with one of four threshold voltage levels, as illustrated in FIG. 3. As a result, a 4-level memory cell can store data having any one of four different values. Therefore, a non-volatile semiconductor memory device (hereinafter referred to as a ‘4-level non-volatile semiconductor memory device’) having such 4-level memory cells may have data storage capability twice that of a non-volatile semiconductor memory device (hereinafter referred to as a ‘2-level non-volatile semiconductor memory device’) having 2-level memory cells.
In a 4-level memory cell, a margin of threshold voltages between neighboring levels may conventionally be about 0.67 V, which is relatively low. Meanwhile, a threshold voltage of a memory cell may shift due to leakage of electrons. Therefore, the threshold voltage of a programmed memory cell MC may shift to a neighboring threshold voltage level. As a result, reliability of a 4-level non-volatile semiconductor memory device may be reduced relative to a 2-level non-volatile semiconductor memory device.